Wave modulator to produce a single sideband signal



April 20, 1954 .1. ENSINK ET AL 2,676,304 WAVE MODULATOR TO PRODUCE A SINGLE SIDEBAND SIGNAL Filed Dec. 11, 1951 INVENTOR Johannes Ensink Jacob Willem %olten By Agent Patented Apr. 20, 195 4 WAVE MODULATOR T PRODUCEYA-SINGLEI. Y i SIDEBAND SIGNAL a J acob Willem Scholten,

Johannes Ensink and Netherlands, assignors to Hartford Trust Company, Hartford,

Hilversum, National Bank'and Conn., as trustee 7 Application December 11.1951, Serial No. 261,098

Claims priority, application Netherlands January 11, 1951 The invention relates to a circuit arrangement for deriving from a carrier-wave modulated by an intelligence signal a single sideband. In known arrangements of this type, the signal is supplied through a first bandpass filter, which only passes signal frequencies, to the input circuit' of a modulaton A carrier wave is supplied to the modulator and is modulated by the signal so that the output circuit of the modulator yields a modulated carrier-wave having upper and lower sidebands. A second bandpass filter passing only frequencies falling within the desired single sideband is coupledto the output circuit to transmit the desired band and prevent transmission of the undesired band.

Such modulator circuits are used frequently in carrier wave telephone systems.

These circuits have a serious disadvantage in that the unwanted sideband, reflected by the output filter, passes through the modulator in opposite sense, thus forming modulation products which cannot pass through the-input filter and are, consequently, reflected there and again pass through the modulator in the initial direction.

gThus modulation products are formed. Some of the frequencies of these products lie within the pass band of the output filter-and are thus transmitted further, which harmfully afiects the transmission.

In the first place the non-ideal properties of such modulator becomes more manifest in the stability and in the divergence from the behavior of similarly built modulators, and secondly, the finite, strongly variable value of the impedances of the filter is found to aiiect greatly the frequency characteristic.

The invention has for its object to providea reduction of these linear distortions produced by reflections at the input and output filters.

The circuit according to the invention has the feature that the modulator comprises a network which constitutes a virtual load for one or more of those sidebands which after at least one reflection at each of the filters, would produce a signal lying within the wanted sideband and occurring across the output circuit.

Consequently in the. circuit according to the invention, the transmission path for the sideband concerned'is cut ofi in the correct manner and 3 Claims. (01. 332- 45) this sideband is dissipated in the load impedance.

For the sake of completeness it should be noted that modulator circuit-arrangements are known in which the modulator comprises a network.

In one of these circuit-arrangements this net- 1 source.

work serves to compensate the natural capacity of the rectifiers of the modulatorand together with this capacity, this network constitutes a resonant circuit, which is tuned to a desired frequency occurring across the output circuit.

A furtherknown circuit-arrangementcomprises a network in the modulator with a view to adapt the modulator-and a bandpass filter comprised in the output circuit. w

In this known circuit-arrangement there is no recognition of the fact that unwanted modulation products can be suppressed by a network in the modulator, whichis used to sift out particular sidebands. I I

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawing.

The input circuit of the modulator I comprises a bandpass filter 2 and the output circuit comprises a bandpass filter 3.

The modulator [is of known type, with the exception of the network to be described hereinafter and is constructed push-pull modulator.

The secondary winding 4 of the input transformer 5 has a center tap 6, which is connected to one of the termin ls l of the carrier-wave supply source.

The primary winding of the output transformer 8 is divided into two parts -9 and Ill. The center taps of these parts are connected through resistors and I2 respectively to the other terminal I, of the The secondary'winding of the transformer 8 is tuned withthe use of a capacitor and forms part of the filter 3.

The lower side of the secondary winding 4 of the transformer 6 is connected through a rectifier I 3 to the lower side of the winding 9 and through a rectifier l4, having opposite polarity, to the upper side of the winding. H).

The upper side of winding 4 is connected through a rectifier l5 oi the same polarity as the rectifier It to the upper side of winding 9 and through a rectifier l6, having opposite polarity, to the lower side of winding l0.

By way of example, it will be assumed that the bandpass filter 2 has a pass band of 44 to 48 kcs. and the filter 3 a' pass band of 452 to 456 kcs., a carrier-wave frequency of 408 kcs. being supplied to the terminals 1.

If a signal is supplied to the input circuit in the range of from 44 to 48 kcs., a number of modulain the form of a double carrier-wave supply tion products is produced, at a carrier-wave frequency of 408 kcs., the principal products being constituted by the upper sideband of the carrierwave lyingbetween. 452-to 456 kcs., and the lower sideband of 360 to 364 kcs., and the two sidebands of the third harmonic of the carrier-wave from 1176 to 1180 kcs., and from 1268 to 1272 kcs.

The first-mentioned sideband is the. wanted sideband and is allowed pass filter 3, whereas the other sidebands, the undesired sidebands, are reflected.

After these undesired sidebands. have passed through the modulator in the reverse direction, some of the modulation products thus produced are reflected at the bandpass filter 2, upon which they pass through the modulator in the initial direction.

One or more of the modulation products thus produced now lie within the pass band of from 452 to 456 120s, of the bandpass filter 3.

In order to suppress one or more of these modulation products, the modulator comprises anetwork, which comprises in this case the series combination of the capacitor l1, inductor IB- and resistor l9.

The rectifiers and the coupling elements mostly exhibit parasitic capacity,.which is designated by 20. This capacity is included in the network, with a View to which the tuning of the capacitor I1 and the coil-l8 are altered accordingly.

The network behaves as av half filter section comprising the input capacity 26, if any, the resonant circuit H, l8 locatedin the longitudinal branch and the resistor [9, included in the transverse branch.

If desired, a capacitor may be connected in parallel with the resistor l9, so-that a complete filter section is formed.

The frequencies of the sideband to which the resonant circuit is tuned are dissipated in the resistor l9.

To further visualize the tuning of this series resonant circuit, it is easier to consider the bandpass filter 3 as the input bandpass filter and the bandpass filter 2 as the output bandpass filter, which is allowable because of the reversibility of modulator operation.

If a signal is supplied to thebandpass filter 3,

which lies in the range of from 452 120-4561 kcs.,

two sidebands occur, 1. e. from 44 to 48 kcs. and from 860 to 864 lacs if modulation products of higher order are left out of consideration.

The first-mentioned sideband is allowed to pass through the bandpass filter 2, but the second is reflected and constitutes the origin of those modulation products which finally produce, in conjunction with the next following reflection at the bandpass filter 3, sidebands which lie within the pass band of the bandpass filter 2.

Consequently, the series circuit I], I8 must be tuned if the capacity 20 is left out of consideration, to a frequency lying within the range of from 860 to 864- kcs.

If the modulator circuit is now viewed in the initial direction, a signal of from 44 to 48 lacs, is supplied, so that the first order sidebands extend from 452 to 456 kcs, and from 160 to 164 kcs. The latter sideband is reflected by the bandpass filter 3 and supplies in the reverse direction as first order products the sidebands of from 44 to 48' kcs., and from 768 to 772 kcs.

to pass through the bandfrom 860 to 864 kcs.,

The first-mentioned sideband is allowed to pass through the bandpass filter 2 and the second is reflected, but in the initial direction it does not supply first-order modulation products lying within the pass band of the bandpass filter 3.

If the reflected sideband of from 360 to 364 kcs., is once more taken as a basis and if modulation products of: high order are also taken into consideration, this reflected sideband is found to produce in the reverse direction a sideband of from 860 to 864 kcs.., in conjunction with the third harmonic ofthe carrier-wave, i. e. with the frequency of 1224 kcs.,,this sideband being reflected by the bandpass filter 2 and producing in the initial direction a first-order modulation product of from 452 to 456 kcs., in conjunction with the carrier-wave, this product falling within the range of the filter 3.

If, as has been set out above, the sideband of is consequently absorbed by the network,,th-is source. of interference has been eliminated.

What we claim is:

l..,A circuit arrangement for deriving from a carrier-wave modulated by an intelligence signal having a predetermined-frequency range a singe sideband, said arrangement comprising a double push-pull diode modulator provided with input and output circuits, a first bandpass filter having a frequency response in which all frequencies falling within said predetermined range are passed and all other frequencies are reflected, means to supply said intelligence signal to said input circuit through said first filter, means to apply said carrier wave to said modulator to combine with said intelligence signal to produce in said output circuit-a modulated carrier-wave having upper and lower sidebands, a second bandpass filter having a frequency response in which all frequencies falling within the frequency range of one of said sidebands are passed and all other frequencies are reflected, and means coupled to said output circuit through said second filter to derive said single band, said modulator further including a resonant network connected across said input circuit and tuned to a selected frequency bandfor dissipating wave energy which has been reflected at least one by each filter.

2. A circuit. arrangement,as set forth in claim 1, in which said network is a series resonant net- Work.

3. A circuit arrangement, as set forth in claim 2, wherein said modulator input circuit includes a parasitic reactance and wherein said tuned network includes in series connection a capacitance and an inductance, said series connection being shunted by said parasitic reactance.

References Cited in the file of this patent 

